The present invention relates to subterranean well construction. More particularly, the present invention relates to methods of isolating particular zones within a subterranean formation utilizing self-degrading cement compositions.
Wells for producing fluids found in subterranean formations may extend several thousand meters below the surface of the earth, and may penetrate several different zones of a subterranean formation. As referred to herein, the term “zone” is defined to mean an interval or unit of rock that is differentiated from surrounding rock on the basis of at least one factor such as, but not limited to, the particular interval's fossil content, fluid content, bulk density, permeability, porosity, compressive strength, tensile strength, shear strength, crystalline structure, or other features, such as faults or fractures. Often, a particular unit of rock may be differentiated from surrounding rock by engineering parameters (e.g., Poisson's Ratio, Shear Modulus, Bulk Modulus, and Young's Modulus) that may be unique to the particular unit of rock. In addition to zones comprising hydrocarbons (e.g., oil and gas), production wells frequently encounter brine and fresh water zones, as well as zones containing undesirable supercritical fluids or gases (e.g., carbon dioxide and hydrogen sulfide). Production wells also may encounter zones containing shales, which may hinder the effectiveness of various subterranean operations, in view of shale's tendency, upon intermingling with some aqueous fluids, to swell and at least partially degrade into clay particles.
To enhance the efficiency of hydrocarbon production from a well, the producing zones (e.g., zones from which hydrocarbons are being, or soon will be, produced) may be isolated from the non-producing zones (e.g., zones from which hydrocarbons or other fluids are not presently intended to be produced) of the subterranean formation. Additionally, it may be desirable to define certain production zones (e.g., more productive zones and/or zones containing a particular fluid to be produced) and isolate them from one another. For example, certain hydrocarbon production zones may produce more sand, water, gas, or wax than other areas. As a result, such sand-, water-, gas-, or wax-producing zones may require maintenance to an extent that may be unnecessary in other zones. As used herein, the terms “isolating a zone” and “zonal isolation” refer to the impairment or prevention of fluid communication between (1) a zone in a subterranean formation and a cased or open well bore penetrating that zone, or (2) at least one zone and another zone in a subterranean formation.
In certain instances, it may be desirable to re-establish fluid communication between zones in a subterranean formation that previously were isolated from each other. For example, during the drilling of a well bore in a subterranean formation, a weakly-consolidated zone (sometimes referred to as a “thief zone”) may be encountered. A thief zone presents a variety of challenges that may increase the difficulty of safely controlling a well. Encountering a thief zone while drilling presents a risk that a portion of the drilling fluid being used to drill the well bore may be lost into the thief zone. Accordingly, it may be beneficial to plug off or isolate the thief zone, at least temporarily, and, after drilling operations have been completed, re-open the zone at a later time (in circumstances wherein the thief zone is located within a larger zone from which hydrocarbons ultimately may be produced).
Conventional attempts to solve this problem have involved, inter alia, the use of plugs that subsequently may be removed from the subterranean formation have been used to accomplish zonal isolation. However, removal of the plug may require additional trips into the well bore, adding cost to the drilling operation. Retrieving the plug by pulling it back up through the well bore may be problematic since there may not be sufficient space within the well bore through which to pull the plug without damaging upper portions of the well bore and/or casing strings set therein.
Another conventional approach to solving this problem includes the use of an acid-soluble plug comprising cement, a salt (e.g., calcium carbonate), and other materials, which, once used, can be dissolved by the introduction of an acidic solution into the well bore. However, this approach may have significant drawbacks, including, inter alia, environmental and occupational safety risks that may result from the use of the large quantities of the acidic solution, the risk that some of the acidic solution may escape into other portions of the subterranean formation, and the delay of waiting for the acidic solution to dissolve the plug. Other conventional approaches involve drilling through the plug, but this may require the use of other drilling equipment (e.g., drilling strings capable of producing a greater force, a stabilizer assembly to keep the drill string from being deflected by the plug) that may further complicate the drilling operation and/or risk damage to the well bore.